West Nile virus (WNV) is an RNA virus that primarily infects birds and culex mosquitoes, with humans and horses serving as incidental hosts. Amplification of virus in a bird-mosquito-bird cycle begins when adult mosquitoes emerge in early spring and continues until fall. This timing coincides with the incidence of disease in humans, which peaks in late summer and early fall. Since it was first detected in New York during 1999, the virus has spread rapidly throughout most of the United States.
Indeed, during the first nine months of 2002, a total of more than 2,500 human cases with laboratory evidence of recent WNV infection were reported in 32 states and the District of Columbia. A total of more than 120 human deaths were reported, with the median age of decedents being 79 years. Additionally, there were reports of nearly 5,000 dead crows and nearly 4,000 other dead birds with WNV infection in the United States. Of more than 3,000 mammals detected with WNV infection, greater than 99% were horses. There were also nearly 3,400 WNV-positive mosquito pools reported.
Most human infections with the virus are not clinically apparent. Overall, only 1 in 150 infections results in severe neurologic illness such as meningitis (inflammation of the spinal cord) or encephalitis (inflammation of the brain). Milder symptoms, which generally last 3 to 6 days and are more commonly reported in connection with WNV infection, include a fever of sudden onset, often accompanied by vomiting, eye pain, headache, myalgia, rash, and lymphadenopathy. The incubation period of WNV, although not precisely known, probably ranges from 3 to 14 days. An analysis of attack rates per million persons during the 1999 New York City outbreak showed that the incidence of severe neurologic disease was more than 40 times higher in those at least 80 years of age when compared with persons up to 19 years of age. Thus, advanced age is an important risk factor for more severe neurologic disease.
In addition to transmission from mosquitoes, transmission has been linked to blood transfusion and organ transplantation. For example, four recipients of transplanted organs from single donor in the U.S. became infected with West Nile virus in mid-2002. Three of the recipients developed encephalitis, with one of the three dying as a result. The fourth recipient developed mild symptoms of viral infection without encephalitis, but also tested positive for the virus. The organ donor, who was injured in an automobile accident, received numerous transfusions of blood products before dying. She was not known to have been ill before the accident, and a sample of her blood taken before any of the transfusions showed no evidence of West Nile virus. In a separate instance, a nursing mother whose breast milk contained WNV and a male liver transplant patient both received transfused blood from a common donor, and both developed West Nile virus infections. A stored blood sample from that donor tested positive for the WNV, again suggesting a common source of the infectious virus.
West Nile virus is a single-stranded plus-sense RNA virus taxonomically classified in the family Flaviviridae, under the genus Flavivirus. Accordingly, the virus is a member of the Japanese encephalitis virus serocomplex, which contains several medically important viruses associated with human encephalitis: Japanese encephalitis, St. Louis encephalitis, Murray Valley encephalitis, and Kunjin virus (an Australian subtype of West Nile virus). The viral genome size is approximately 11 kb.
Nucleic acid-based tests for WNV have been described. For example, Shi et al., in J. Clin. Microbiol. 39:1264 (2001) have described a real-time polymerase chain reaction (PCR) assay for WNV nucleic acids. Lanciotti et al., in J. Clin. Microbiol. 38:4066 (2001) have described a TaqMan-based assay for the detection of WNV RNA in human specimens, mosquito pools, and avian tissue specimens. Despite the availability of these PCR-based tests, there remains a need for a WNV screening assay that is specifically adapted for the needs of clinical testing laboratories. The method should particularly lend itself to high throughput screening which may be required for testing large numbers of clinical and donated blood or tissue samples.